Lava started flowing out of the 7,746-foot Mount Karthala at 1:30 a.m., hours after the volcano started spewing ash and dark smoke over Grand Comore, the largest of the three Comoros islands, said Bernadette Ninyaratunga of the United Nations Children's Fund.
A team of experts flew over the volcano to assess the dangers and the damage caused so far, said Maj. Salimou Mohamed, a spokesman for the government's emergency team.
Comoros, one of the world's poorest countries, is about 185 miles east of the African nation of Madagascar.
Nobody was injured and no damage was reported after the eruption, Mohamed told The Associated Press.
"The only problem that is going on at present ... is a rain of ashes," Mohamed said. "These ashes are widespread, I can say it is countrywide."
Mohamed said that in the short term, it was "very unlikely" the explosion of lava at the top of the volcano would affect other areas.
All flights to the island nation were canceled Monday afternoon.
Mount Karthala lies at the center of the southern half of Grande Comore, and the capital of Moroni, with a population of about 50,000, sits at the foot of the mountain's western slope.
Emergency officials worried that the volcano may spew sulfur and other toxic gases. Maanfou said the government had asked the Comoros Red Crescent to request gas masks from its partners.
But Mohamed said the skies were clear of ashes later Monday and authorities would not prevent people from returning to their homes on the slopes of the volcanic mountain, although he said no official announcement was being made.
"This is a volcano, you cannot predict that this is the end of danger and there is no problem," he said.
Some 10,000 people from the area have taken shelter with relatives in several towns, including the capital, said Mohamed Maanfou, deputy secretary general of the Comoros Red Crescent.
The volcano last erupted in July 1991. No one was killed then, but tens of thousands of villagers left their homes.
While the volcano spewed ash and smoke before the eruption, officials urged residents to leave eastern parts of Grand Comore and mobilized trucks and other vehicles to help evacuate people, Giuseppina Mazza, head of the U.N. team in the Comoros, said Sunday.
Medical teams also were dispatched to aid those who had breathing problems because of the ash, she said.
The lava was flowing to the eastern and probably southeastern slopes of the mountain, Maanfou said by telephone from Moroni.
Comoros, a republic that gained independence from France in 1975, has a population of about 770,000 and few natural resources.
By Rodrique Ngowi